This invention relates generally to heterojunction bipolar transistors (HBTs) and more particularly to high power HBTs.
As is known in the art, heterojunction bipolar transistors (HBTs) have been used in a wide range of applications. Group III-V HBTs, such as gallium arsenide (GaAs) based HBTs are particularly suitable for use in high power microwave applications, such as high power microwave amplifiers. One N type HBT includes a GaAs semi-insulating substrate having epitaxially grown thereon a sequence of overlaying III-V layers; i.e., an N.sup.+ sub-collector layer, an N.sup.-, collector layer, a relatively thin heavily doped P.sup.+ base layer, a high bandgap N type emitter layer forming a heterojunction with the base layer, and a N.sup.+ emitter contact layer, respectively. The N.sup.+ emitter contact layer may be GaAs. The high bandgap N type emitter layer may be aluminum gallium arsenide (AlGaAs) or indium gallium phosphide (InGaP).
In one type of high power heterojunction bipolar transistor 6, a plurality of transistor cells 7 are electrically interconnected, as shown in FIG. 1. More particularly, each cell 7 includes a base electrode B, an emitter electrode E, and collector electrode C. The emitter electrodes, E, of the cells 7 are connected together in a grounded, or common emitter configuration, as shown schematically in FIG. 1. In order to equalize power dissipation among the emitters, ballast resistors, R, may be included between each emitter electrode and ground, as shown. The collector electrodes, C, are connected together at a collector contact CC and the base electrodes, B, are connected together at an input signal, or base contact BC, as shown in FIGS. 1 and 2. The emitter electrodes, E, are connected to a ground plane conductor, not shown, disposed on the bottom of the substrate by conductive vias CV which pass through the substrate. It is noted that base, emitter and collector electrodes are end-fed; that is the electrodes B, E and C are fed at ends E1, E2, and E3, respectively, as shown.